A term used in the discussion of rhetorics.
It means rhetoric gone wrong. The use of non-standard words, grammar, spelling, or anything else having to do with words.
Putting letters in the rong plase, using a lingua franca (or lingua ignota, for that matter), a line that doesn't fit into the meter of a poem, messing up your HTML, or even a mispronounced word. Any of this can be considered barbarism.
If you do any of this for aesthetic reasons, it's called a Metaplasm.